We propose to synthesize biologically active steroid hormones in which hydrogen in specific single and multiple positions has been totally replaced by deuterium. When the introduced deuterium is in a region of attachment of the steroid molecule to the active or receptor site of an appropriate protein, significant changes in association constants can be expected to occur and have been found in a preliminary experiment. Isotope substitution at positions not near the region of protein contact are unlikely to have an appreciable effect. This technique should allow detailed mapping of the portions of steroid molecules directly in contact with their receptor or active sites. If it can be shown that different hormone effects are due to interactions of given steroid with different receptor sites, the technique of partial deuterium substitution may offer a means of increasing the effectiveness of a steroid in cancer chemotherapy while leaving unenhanced undesirable side effects, or of reducing side effects while leaving the intended therapeutic use unchanged. This principle may be generally applicable in drug therapy.